Stacked stripline circuits

ABSTRACT

The present invention provides a method and apparatus for design of low loss, size restricted high frequency circuits. In a preferred embodiment, an electronic device includes: a first circuit layer located above the main circuit board comprising a first stripline passive circuit; and a second circuit layer located above the first circuit, the second layer comprising a second stripline circuit. The two stripline circuits can be separately coupled to leads, or coupled to each other and other leads using vias through the ground layer(s) separating each stripline. The stacked stripline elements can be used together with other circuits, and the stacked circuit board can be conveniently joined together with other assemblies, e.g., by surface mounting to a main board. The utility of this topology can be extended by the use of n-circuit embodiment or embedding in a multilayered main circuit board.

FIELD OF THE INVENTION

The present invention is directed to radio frequency or microwave communications systems or subsystems that require integration of multiple passive circuits in limited spaces.

BACKGROUND OF THE INVENTION

Passive elements are used in all manner of electronic circuits. A wide variety of implementations are known for implementing passive circuits, including lumped element and distributed element designs. In certain applications microstrip and stripline designs have also been employed. For example, a stripline is a printed signal path disposed between two ground planes in a printed circuit board. Many single-element stripline circuits are known and used in the industry (e.g., as implemented by the Anaren Xinger couplers), and disclosures have been made about extending a single element stripline into two layers (see, e.g., U.S. Pat. No. 5,929,729 to Swarup (where coupling was an objective), and U.S. Pat. No. 5,359,304 to Fujiki.

In certain high frequency applications, prior approaches towards designing passive elements have proved unsatisfactory. For example, lumped-element harmonic filters tend to result in poor manufacturing yield, and board-to-board performance variation is typical, requiring expensive hand-tuning. On the other hand, distributed element designs are not typically used where there is limited board space and the possibility of re-entrant bands (e.g., passbands repeated at harmonic frequency, to suppress out-of-band rejection). If the application has to be low loss, the design constraints become even more difficult. For example, where the radio-frequency (RF) is above 1 GHz, many applications will require limiting the radiation of unintended signals (spurs, harmonics, etc) to maximize frequency spectrum reutilization. Such requirements drive the need for passive circuits like low pass filters that provide the lowest possible insertion loss and maximum out-of-band rejection. The conventional lumped-element elliptic low pass filters are very difficult to implement at frequencies above 1 GHz due to small component values and extreme sensitivity to component tolerance. Stripline circuits would not be considered by a typical designer, since the physical dimensions of distributed elements at frequencies below 2 GHz are prohibitively large. This rules out the use of distributed element filters when more than one filter has to be implemented in a constrained board space.

There remains, therefore, a need for a better approach to circuit design for multiple high frequency (i.e., greater than 1 GHz), low loss passive elements for use in constrained-size applications. Just such an approach is now possible by the invention described in more detail in connection with the following embodiment.

SUMMARY OF THE INVENTION

The present invention provides such a method and apparatus for design of low loss, size restricted high frequency circuits. In a preferred 2-circuit embodiment, an electronic device includes: a first circuit layer located comprising a first stripline passive circuit between a top and a bottom ground layer; and a second circuit layer located above the first circuit, the second layer comprising a second stripline circuit between a top and a bottom ground layer; the bottom ground layer is shared with the first circuit. The two stripline circuits can be separately coupled to leads, or coupled to each other and other leads using vias through the ground layer(s) separating each stripline. The stacked stripline elements can be used together with other circuits, and the stacked circuit board can be conveniently joined together with other assemblies, e.g., by surface mounting to a main board. An N-circuit implementation extends this 2-circuit embodiment over more circuit layers.

BRIEF DESCRIPTION OF THE DRAWINGS

While the invention is defined by the appended claims, as an aid to understanding it, together with certain of its objectives and advantages, the following detailed description and drawings are provided of an illustrative, presently preferred embodiment thereof, of which:

FIG. 1 is an illustration of a 2-circuit stripline implementation of 2 different filters according to a first embodiment of the present invention.

FIG. 2 is an illustration of an N-circuit stacked stripline implementation of multiple stripline circuits integrated into a single package.

FIG. 3 is an illustration of an alternate implementation of the present invention where the stacked stripline circuit is part of the main circuit board which has multiple layers.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In a preferred embodiment of the invention, a system is provided for high frequency passive elements, where plural passive elements are formed in stripline topology stacked vertically in relation to each other. This arrangement permits implementation of multiple high frequencies, low loss circuits in a small footprint.

Turning now to FIG. 1, there is shown a cross section of a 2 layer stripline circuit of package 100 for use according to a first embodiment of the invention. The circuit package includes 2 circuit layers. Each circuit layer includes a stripline circuit for performing a unique function such as filtering a signal.

The circuit layer 130 is preferably a stripline circuit 132 sandwiched between ground planes 133 and 134. The stripline circuit 132 is isolated from the ground planes 133 and 134 by dielectric substrates 135 and 136, respectively. The stripline circuit 132 is connected to the circuit on the main board 110 by a signal via line 131. Similarly, the second circuit layer 140 comprises a circuit 142 sandwiched between substrate layers 145 and 146, and ground layers 134 and 144. The stripline circuit 140 is connected to signal trace on the main board 110 by via connections 141. Ground plane 133 is connected to the mainboard 110 by solder connections 107. The stripline topology provides power handling capability better than microstrip and built-in shielding.

Below the mainboard 110, FIG. 1 also shows a bottom view of the stacked stripline assembly 133 (looking up from the mainboard 110). This view shows the locations of the via connections 111, 131 and 141 in the interior of the element 133.

In a 2-circuit embodiment of the invention we implement two elliptic low pass filters in stripline structure in a single package, again with one filter “stacked” on top of the other. In this embodiment, stripline circuit layer 130 comprises the first elliptic low pass filter and stripline circuit layer 140 comprises the second elliptic low pass filter, each having different cut-off frequency.

By stacking two distributed-element filters thus, it is possible to have both filters in the space normally required for one filter. The ground plane between the two filters, an integral part of stripline circuit structure, provides isolation between the two filters. This also eliminates the need to provide external shielding which would add additional parts to the end-assembly. In addition, the use of a distributed element filter allows for more repeatable performance compared to lumped-element filters.

The distributed element filters are synthesized from the lumped-element circuit model using available filter element value tables. To optimize the performance of the filters, electromagnetic simulation is used to tune the dimensions of prototype filters. The filter using multiple cascaded hairpin resonators provides a very sharp cutoff frequency response with low insertion loss. Furthermore, to increase the rejection-band bandwidth, additional attenuation poles are added to the filter. The filters are evaluated by building a few prototypes and ascertaining that the measurement is in agreement with the simulation. This embodiment realizes a wide stop-band analogous to the ideal lumped element version. The measurement shows that the second filter (on circuit layer 130) had typically 30 dB up to 14 GHz and 25 dB out-of-band rejections up to 18 GHz. The cut-off frequency of the second filter is 2 GHz. The circuit can be designed as a separate board that can be surface-mounted to the main board 110.

This embodiment may be used to replace two different elliptic low pass filters used in an RF (radio frequency) transmitter, particularly where very limited board space is available. The lumped-element version may comprise 20 different inductors and capacitors. The performances of the lumped-element version could vary substantially due to component value tolerances. Tin shields might also be required to mitigate the performance degradation caused by coupling between elements and the two filters. One integrated component in accordance with the above embodiment with dual function; may thus replace 20 pick-and-place components with one and eliminate the need for separate RF shielding.

The utility of this embodiment is not limited to two circuits or filters. More than two passive circuits realizable in stripline structure can be integrated in a single package, such as shown in FIG. 2. FIG. 2 shows a cross sectional view of an N-layer implementation wherein a stacked stripline circuit assembly 120 is attached to the main board with a solder connection 107. As an example, additional filters 150 can be added to the 2-circuit embodiment by simply stacking more filters. Other passive circuits such as couplers, power splitters, or delay lines can be implemented in one of the layers. Some active circuits can also be implemented, such as phase shifters, attenuators, and switches, etc. The active elements can be installed in pockets in the inner layers or, preferably, on the top layer with via connection to other layers. These can be implemented as a single integrated circuit (IC), or part of a series of smaller ICs, (e.g., subsequently surface mounted or otherwise joined to a main board of a component or electronics device). Further, it may also be implemented as part of a multilayer circuit board, such as illustrated by FIG. 3. This embodiment may be used when the circuit implemented in FIG. 1 or 2 will not require subsequent changes. By doing so, a soldering step can be eliminated and an extra main circuit board space can be freed for use by other circuits.

FIG. 3 shows an embedded implementation of an N-layer stacked stripline circuit as part of an N-layer board 211. An embedded stacked stripline circuit 220 comprises a first circuit 230, a second circuit 240, up to an Nth circuit 250 with a top layer 271. Other mainboard circuits 270 are spread among the layers.

Of course, one skilled in the art will appreciate how a variety of alternatives are possible for the individual elements, and their arrangement, described above, while still falling within the scope of the invention. Thus, while it is important to note that the present invention has been described in the context of an implementation for plural stripline filters, those of ordinary skill in the art will appreciate that the present invention applies equally regardless of the particular type of passive element actually used in implementing the desired circuit. Further, while this has been found particularly useful for circuits in the 1-2 GHz range, and more generally in the 1-4 GHz range, it should have equal application to all high frequency (i.e., greater or equal to 1 GHz) integrated circuits depending on the desired application and other design considerations.

In conclusion, the above description has been presented for purposes of illustration and description of an embodiment of the invention, but is not intended to be exhaustive or limited to the form disclosed. This embodiment was chosen and described in order to explain the principles of the invention, show its practical application, and to enable those of ordinary skill in the art to understand how to make and use the invention. Many modifications and variations will be apparent to those of ordinary skill in the art. Thus, it should be understood that the invention is not limited to the embodiments described above, but should be interpreted within the full spirit and scope of the appended claims. 

1. An electronic device, comprising: a first high frequency circuit layer located above a circuit board comprising a first stripline passive circuit; and a second high frequency circuit layer stacked above the first high frequency circuit layer, the second layer comprising a second stripline passive circuit, coupled to a signal trace on the circuit board by at least one via.
 2. The device of claim 1, wherein the first stripline circuit comprises a signal trace layer and a first ground layer located below the signal trace layer and a second ground layer located above the signal trace layer.
 3. The device of claim 2, wherein the first and the second stripline circuit is one of a group of a filter, a power divider, a coupler, and a delay line.
 4. The device of claim 1, wherein the device is a high frequency assembly of stacked stripline elements adapted for surface mounting to a further electronics board.
 5. The device of claim 4, wherein the device is a high frequency integrated circuit of stacked stripline elements operable in the range of 1 to 4 GHz.
 6. A method for making an electronic device comprising: a. preparing a circuit board of predetermined dimensions for layering stacked circuits above a board substrate, comprising forming a ground layer and signal traces for connection to the stacked circuits and adapted for coupling with conductive elements not part of the device; b. layering a first high frequency stripline passive circuit above the circuit board, and coupling the first circuit to first signal launch points on the outer ground layer by a plurality of vias from the first circuit layer; and c. layering at least a second high frequency stripline passive circuit above the first circuit and coupling the second stripline circuit to second signal launch points on the outer ground layer by vias from the second circuit layer; whereby the first and second circuits form a 2-circuit stacked stripline integrated circuit.
 7. The method of claim 6, wherein the first stripline circuit is layered so as to comprise a signal trace layer and a first ground layer located below the signal trace layer and a second ground layer located above the signal trace layer.
 8. The method of claim 6, wherein the second stripline circuit is layered so as to comprise a second signal trace layer above the second ground layer and a third ground layer above the second signal trace layer, whereby the second ground layer forms a ground.
 9. The method of claim 6, wherein the first stripline circuit is layered so as to form one of a group of a filter, an inductor, a transformer, a phase shifter, an attenuator, and a switch.
 10. The method of claim 6, wherein the first and second stripline circuits are layered so as to form a high frequency integrated circuit of stacked stripline elements adapted for surface mounting to a further electronics board.
 11. The method of claim 6, wherein the first and second stripline circuits are layered so as to form a high frequency integrated circuit of stacked stripline elements operable in the range of 1 to 4 GHz and adapted for surface mounting to a further electronics board.
 12. The method of claim 6, wherein the first and second stripline circuits are layered so as to form a high frequency integrated circuit of stacked stripline elements operable in the range of 2 to 3 GHz and adapted for surface mounting to a further electronics board.
 13. The method of claim 6, wherein at least one of the first and second stripline circuits is layered so as to form elliptical filters comprising multiple cascaded hairpin resonators with plural attenuation poles.
 14. An electronic assembly comprising: a main board; and an integrated circuit surface mounted to the main board, the integrated circuit surface comprising: a. a circuit board; b. a first high frequency stripline passive circuit layer located above the circuit board comprising a signal trace layer and a first ground layer located below the signal trace layer and a second ground layer located above the signal trace layer; and c. a second high frequency stripline passive circuit layer stacked above the first high frequency circuit, the second layer comprising a second signal trace layer above the second ground layer and a third ground layer above the second signal trace layer, whereby the second ground layer forms a ground layer for both the first and second circuit; wherein the stripline circuits each form at least one of a group of a filter, an inductor, a transformer, a phase shifter, an attenuator, and a switch, and are operable in the range of 1 to 4 GHz.
 15. An electronic circuit board device comprising plural high frequency circuit layers each comprising a stripline passive circuit, wherein a first high frequency circuit layer comprises a first stripline passive circuit; and a second high frequency circuit layer stacked adjacent to the first high frequency circuit layer comprises a second stripline passive circuit, having a ground layer between the first and second stripline passive circuits; and subsequent high frequency circuit layers stacked adjacent to the second layer in a same manner.
 16. The device of claim 15, wherein the plural high frequency circuit layers are embedded in a passive circuit region in the electronic circuit board device.
 17. The device of claim 15 further comprising an active element located in one of a group of within the passive circuit region and above a top layer of the passive circuit region.
 18. The device of claim 15, wherein the active element together with at least one of the stripline passive circuits form one of a group of a filter, an inductor, a transformer, a phase shifter, an attenuator, and a switch. 